The Israeli military killed six Palestinians, including a baby, who were in a school that sheltered displaced people in Gaza City on Friday, hospital officials have said. The attack brings the number of Palestinians killed by Israel to 401 since the October ceasefire took effect.

Israeli troops fired over the ceasefire line which they have withdrawn to, killing the Palestinians and wounding a number more, the Palestinian civil defence said in a statement. It added that it was only able to recover the bodies after coordinating with the UN, to ensure they did not also come under Israeli fire.

Dozens gathered at Gaza City hospital to mourn those killed, with five body bags placed outside the hospital and a man cradling an infant wrapped in a white shroud.

“This is not a truce, it is a bloodbath,” said Nafiz al-Nader, who witnessed the attack, telling Agence France-Presse he wanted the bloodshed to stop. Another witness said shelling erupted without warning on Friday night, hitting the school.

The ceasefire line refers to where Israeli troops have pulled back to under the terms of the October ceasefire deal, denoted by a long yellow line on maps and physically demarcated by yellow concrete markers on the ground. Israeli troops remain in control of about 53% of the territory and regularly conduct airstrikes in the areas it does not occupy.

Friday’s killings are the latest in a series of challenges to the ceasefire, now in its third month, which mediators are trying to push towards a second phase. The first phase of the ceasefire called for an end to hostilities between Hamas and Israel, the withdrawal of Israeli troops to the yellow line, and the increase of humanitarian aid into Gaza, which had slipped into famine earlier this year due to Israeli aid restrictions.

The second phase of the ceasefire is meant to lead to a permanent peace in Gaza, but mediators must first settle differences between Hamas and Israel on thornier issues. Israel stipulates that Hamas give up its arms and surrender power to a civilian transitional authority, while an international stabilisation force is deployed in Gaza, and Israel is meant to fully withdraw from Gaza.

Relatives of the Palestinians who died mourn as bodies are brought to the al-Shifa hospital for burial in Gaza City on 20 December. Photograph: Anadolu/Getty

There is no clear agreement between the parties on how to bridge the gaps between Hamas and Israel on those points. On Friday, the US secretary of state, Marco Rubio, said the mandate of the international stabilisation force – a key part of the ceasefire plan – would need to be clarified before foreign countries make troop commitments.

At a press conference, Rubio said: “In fairness to all the countries we’ve talked to about having a presence on the ground, I think they want to know specifically what the mandate will be and what the funding mechanism looks like,” adding that there were a “number of nations states acceptable to all sides” willing to participate in the international force.

The ceasefire, which has prevented the resumption of all-out funding, has grown increasingly shaky as momentum for a permanent end to the two-year Gaza war stalls.

Qatar’s prime minister, Sheikh Mohammed bin Abdulrahman bin Jassim al-Thani, warned on Thursday that further delays in moving to the second phase of the deal “endangers the entire process”.

The war in Gaza started after Hamas-led militants killed about 1,200 people and took 251 hostage on 7 October 2023. All but one of the hostages and their remains have been returned to Israel in exchange for Palestinian detainees and prisoners under the ceasefire deal.

More than 70,925 Palestinians have been killed in Israel’s war on the territory, around half of whom were women and children, according to the Gaza health ministry. The death toll is expected to rise as more bodies are found under the rubble.

Much of the strip’s civilian infrastructure and homes were levelled by Israeli bombing.

A UN commission, Amnesty International and Human Rights Watch said that Israel has committed genocide against Palestinians in the Gaza Strip – a claim that Israel has denied.